The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), in Parashat Vayigash (1; listen to audio recording for precise citation), discusses the laws pertaining to the recitation of the verse, "Hashem Melech" in the prayer service. He writes that whenever we recite this verse, we recite it twice. This verse expresses our acceptance of God as King over us, and we recite it twice to emphasize that we accept the Almighty’s kingship over both our bodies and our souls. (The Ben Ish Hai cites a passage from the Zohar as the source for this Halacha.) Precedent for this practice is found in the Book of Melachim I (18:39), where we read that Beneh Yisrael declared their loyalty to God by declaring, "Hashem Hu Ha’Elokim Hashem Hu Ha’Elokim" ("Hashem is God, Hashem is God"). Just as Beneh Yisrael repeated this declaration, so must we repeat "Hashem Melech" in order to express our commitment of both body and soul to the Almighty.
The Ben Ish Hai further writes that one must stand while reciting "Hashem Melech." In fact, even if one is reciting Shema or another part of the prayer service when the congregation recites "Hashem Melech," he should stand for the recitation. One should not, however, interrupt his prayer to join in the recitation of "Hashem Melech." He simply stands together with the congregation. This applies even if one recites a part of the prayer service where Halacha allows interruptions, such as the Akeda or the Korbanot. Although he must stand together with the congregation, he should not interrupt his recitation to recite "Hashem Melech." Certainly, however, if one is not in the middle of prayer, but is rather learning or just sitting in the synagogue, he should stand and join in the recitation of "Hashem Melech." This applies also to the recitation of "Hashem Melech" during the Selihot service.
The exception to this rule, the Ben Ish Hai writes, is the recitation of "Hashem Melech" during Hakafot on Simhat Torah. As that recitation is merely a Minhag (custom), and not required according to the strict Halacha, one who is in the synagogue during Hakafot and hears "Hashem Melech" is not required to stand or join in the recitation.
Finally, the Ben Ish Hai writes that an adult must lead the recitation of "Hashem Melech." In congregations where minors are invited to lead the Pesukeh De’zimra service, an adult must replace the minor for the recitation of "Hashem Melech." The Ben Ish Hai emphasizes that congregations in which minors are allowed to lead the recitation of "Hashem Melech" must change their practice and see to it that specifically an adult leads this important part of the prayer service.
Summary: When we recite "Hashem Melech" in the prayer service, we recite it twice, and we stand for this recitation. If one hears the congregation reciting "Hashem Melech" while he prays a different part of the service, he should rise with the congregation, though he should not interrupt his prayer. Minors should not be allowed to lead the recitation of "Hashem Melech," even in congregations which allow minors to lead the Pesukeh De’zimra service.
